RECOMBINATION AND DISSOCIATION OF H2+ AND H3+ IONS ON SURFACES TO FORM H2(V'') - NEGATIVE-ION FORMATION ON LOW-WORK-FUNCTION SURFACES

The recombination and dissociation of H+2 and H +3 ions incident upon metal surfaces leads to H, H 2(v'), and H- products rebounding from the surface. A four-step model for H+2 -ion recombination generates H2(v') via resonant electron capture through the b 3Σ+u and X 1Σ +g states. A molecular trajectory analysis provides final-state H2(v') distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2@B: H+2 yields compare favorably with the observed yields. A similar four-step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E'→2p 2A1) ground state, in turn dissociating into H+H2(v'), with the fragment molecule rebounding to give the final H2(v') distribution. Comparing the final populations v'≥5 for incident H+2 or H +3 shows that the H+3 ion will be more useful than H+2 for H- generation via dissociative attachment. Molecular ions incident upon low-work-function surfaces generate additional H2(v') via resonant electron capture through excited electronic states and provide two additional sources of H- production: Direct H- production by H dissociation products rebounding from the surface and H- production through the formation of H-2 in the surface selvage that in turn dissociates into H+H-. The H-2 in the selvage is formed by resonant capture to the low-lying vibrational levels of H2(v'), and complements dissociative attachment to high-lying levels in the discharge. The H, H2(v'), and H- yields are inventoried for H +3 incident upon barium surfaces.